A fellow agent and close friend, Charlie Merriweather (Clark Johnson), is murdered. Garrison gets word from a trusted informant that the killing of Merriweather is related to an assassination plot against the President. The intelligence provided by the informant reveals that a mole with access to the President's security detail had provided information to the assassins.

The Secret Service Protective Intelligence Division, led by Garrison's former protégé and ex-friend David Breckinridge (Kiefer Sutherland) with rookie partner Jill Marin (Eva Longoria), is tasked with investigating the plot, which begins with polygraphs for every agent. Meanwhile, the mole discovers the discussion with the informant and Garrison's affair with the First Lady, and attempts to blackmail Garrison by luring him to a coffee shop known to be a meeting point for Colombian gangs. After failing the polygraph test (for lying about the affair), Garrison becomes the prime suspect for providing the information to the assassins.

Breckinridge confronts Garrison at home and begins to interrogate him. The source of rancor between them comes to light: Garrison supposedly had an affair with Breckinridge’s wife and caused the breakup of their marriage, which he denies. Garrison escapes capture and conducts an independent investigation of the assassination plot, while making brief contact with the First Lady to deny his involvement. He tries to contact the informant who gave him the tip, but finds that he has been killed.

Breckinridge gets the drop on Garrison but refuses to kill him, despite giving other agents "shoot to kill" orders. Using his contacts with sympathetic agents and family members, Garrison tracks down the location of one of the assassins, and learns they are headed to Toronto to attack the president at the G8 summit. After killing the assassin and finding incriminating evidence in the apartment, Garrison tells Marin of the discovery, but later finds that the evidence and body of the assassin was removed before she arrived.

The First Lady discloses her affair with Garrison to Breckinridge, who now believes that Garrison is innocent. Together in Toronto, they discover the identity of the assassins and the mole, senior agent William Montrose (Martin Donovan), who was never polygraphed. Montrose is in charge of directing security at the G8 summit.

The leader of the assassins, The Handler (Ritchie Coster), tells Montrose to give him the President. Montrose attempts to refuse, but there is a threat to the lives of the agent's wife and two daughters if he backs out. Emotionally torn, Montrose is instructed by The Handler to jam the secret service's communication radios, and leave the summit via a specific route; the assassins will handle the rest.

On the night of the President's speech, Breckinridge and Garrison race to the summit. The assassins, dressed as Royal Canadian Mounted Police (RCMP) ERT operators, kill several agents and try to kill the President. Montrose reveals he is the mole to the President, despite the assassin's threats against Montrose's family. Montrose then purposely walks in front of and is ultimately killed by one of the assassins in the stairwell.

Garrison, Breckinridge and Marin kill the remaining disguised assassins while safely bringing the President and First Lady up to the summit to be evacuated. With all of his accomplices dead, The Handler comes forward dressed as an RCMP officer personally to kill the President and First Lady. He grabs Sarah hostage and aims his pistol to shoot the President, but Garrison shoots him dead.

In spite of the events, Garrison is forced to take an early retirement due to the disclosure of the affair with the First Lady, who looks on sadly from her window as Garrison leaves the White House. He does, however, make peace with Breckinridge, who finally realizes that Garrison never slept with his wife. Breckinridge tells Garrison that he is meeting her later on to talk things over.

The film received generally poor reviews, scoring 34% on Rotten Tomatoes,[3] the site's consensus reading "The Sentinel starts off well enough but quickly wears thin with too many plot holes and conventional action sequences." The BBC review described it as being "as compelling as watching the ink dry on a superfluous UN treaty".[4] Some other reviewers, such as one from the Los Angeles Times, enjoyed the film.[5]